A conventional contactless switch of the kind which the invention seeks to improve is shown in FIG. 4. In FIG. 4, an AC power supply 1 and load 2 are connected in series, across the terminals T.sub.1 and T.sub.2. The input terminals of a single phase bridge rectifier circuit 3 are connected between these terminals T.sub.1 and T.sub.2, and a thyristor 4 is connected across the output terminals of this rectifier circuit 3. This thyristor 4 provides a switching operation and open and closes the series circuit consisting of the power supply 1 and the load 2 through the rectifier circuit 3. In parallel with the thyristor 4, there is connected a series circuit including a series type constant-voltage circuit 8 consisting of a transistor 5, Zener diode 6, and a resistor 7, and a transistor 11. As shown, the resistor 7 is connected between the base and collector of transistor 5 and the collector of transistor 11 is connected to the base of transistor 5 by way of the Zener diode 6. The emitter of transistor 5 and the emitter of transistor 11 are connected to the input terminals of a sensor circuit 10 which includes a detection coil 9. Sensor circuit 10 is of conventional design, such as the proximity switch Model TCA 205 commercially available from Siemens AG, in which the inductance of detection coil 9 changes when an intruder, for example, or other object approaches the coil. The input terminals of the sensor circuit 10 are connected through the constant-voltage circuit 8 across the thyristor 4. When the Zener voltage of the Zener diode 6 is at V.sub.Z1, the voltage drop between the base and the emitter in the transistor 11 when a saturation current is flowing is V.sub.CE. The constant-voltage circuit 8 applies the constant voltage of V.sub.Z1 +V.sub.CE -V.sub.BE to the sensor circuit 10. The sensor circuit 10 turns the transistor 11 off by changing its output signal, which has been at a high level, to a low level when an object to be detected approaches the detection coil 9, thus disabling the constant-voltage function of the constant-voltage circuit 8. Reference numeral 12 indicates a capacitor connected in parallel with the sensor circuit 10. A gate circuit 15 consisting of a series circuit including Zener diode 13 and resistor 14, is connected in parallel with the capacitor 12, and the node between the Zener diode 13 and the resistor 14 is connected to the gate of the thyristor 4. The Zener voltage V.sub.Z2 of the Zener diode 13 is selected so that V.sub.Z2 +V.sub.GK &gt;V.sub.Z1 +V.sub.CE -V.sub.BE, where V.sub.GK is the threshold gate-to-cathode voltage for turning on the thyristor 4. Since under this condition the Zener diode 13 is not conducting and the gate current of the thyristor 4 does not flow, the thyristor 4 is kept off.
When the subject approaches the detection coil 9, the output signal of the sensor circuit 10 assumes a low level and the transistor 11 turns off. Thus, the function of the constant-voltage circuit 8 is disabled and the current flowing in the resistor 7 becomes the base current of the transistor 5, resulting in an increase of the voltage across the capacitor 12. Accordingly, current flows to the gate of the thyristor 4 through the Zener diode 13 of the gate circuit 15, which turns the thyristor 4 ON and causes conduction through the load 2. When the thyristor 4 is ON, the voltage across its anode-cathode terminals is low, the current supply from the power supply 1 to the sensor circuit 10 is discontinued, and the sensor circuit 10 continues its detecting operation while consuming the charge of the capacitor 12. The thyristor 4 which is ON is turned OFF at each zero point of the current at the power supply frequency, which, however, has little effect. Since the capacitor 12 is being discharged while supplying current to the sensor circuit 10, its voltage eventually becomes lower than the Zener voltage V.sub.Z2 of the Zener diode 13, the Zener diode 13 becomes nonconducting, and the thyristor 4 turns OFF. However, as the voltage across the terminals of the thyristor 4 rises, the transistor 5 turns ON and the capacitor 12 is charged. The thyristor 4 turns ON again when the voltage of capacitor 12 reaches the Zener voltage V.sub.Z2 of the Zener diode 13. Thus, while an object is close to the detection coil 9, the thyristor 4 turns OFF momentarily at the zero point of AC full-wave rectification, but the effect of the whole switch is an ON status.
However, because the voltage of the capacitor 12, the driving voltage of the sensor circuit 10, has not risen to the voltage needed for operating the sensor circuit 10 normally when the power is initially applied, the output of such AC contactless switch is unstable, whether the detection signal is at high level or low level. Thus a malfunction is liable to occur, and the rectifier circuit and the thyristor apt to be damaged, when the load is short-circuited because of the failure of the load. Accordingly, it has been the tendency to protect the rectifier circuit and the thyristor by providing an overcurrent detection circuit in the thyristor circuit. However, the thyristor, once triggered ON, maintains the conductive status until the next current zero point. Thus a circuit of large capacity for withstanding such overcurrent is required and a resistor for limiting the current is usually needed to be connected in series. This causes useless power consumption.
An object of the present invention is to provide an AC contactless switch capable of protecting against overcurrent at the turning on of the power supply or, because of short-circuiting of the load.